Chapter 9 - Enthalpy (MODULE 3) Flashcards
What is enthalpy
- what is the symbol for enthalpy
a measure of the heat energy in a chemical system
- H
What is the enthalpy change notated as
- how can it be worked out
- delta H
- enthalpy of products - enthalpy of reactants
if enthalpy change is negative what type of reaction is it
exothermic
if enthalpy change is positive what type of reaction is it
endothermic
What does the law of conservation of energy state
Energy cannot be created or destroyed
What does the reaction profile have on its axis
- what is the distance between the start and the highest point known as
- what is different about endothermic and exothermic reaction profiles
H, time
- Activation energy
- endothermic reactions have reactants higher than products while exothermic has reactants below products
What is the activation energy
the minimum energy required to break the initial bonds
What happens during chemical reactions
bonds are broken and then new bonds are formed
What symbol is used to show standard conditions when talking about enthalpy
H°
What are standard conditions for enthalpy changes
100kPa
298K
1 mol dm^-3
standard state at temperature and pressure
What is the standard enthalpy change of a reaction
the enthalpy change that accompanies a reaction in the molar quantities shown under standard conditions
What is the standard enthalpy change of formation
the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
What must be done before calculating the standard enthalpy of formation / combustion
make sure that one mole of the compound involved is formed / used
What is the enthalpy change of formation of elements
0
What is the standard enthalpy change of combustion
the enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
What is the standard enthalpy change of neutralisation
the energy change that accompanies the reaction of an acid with a base to form one mole oh H2O (l)
What is special about the enthalpy change of neutralisation
it is the same for all neutralisation reactions
How many Kelvin is 0 degrees celsius
- how many degrees celsuis is 0K
273K
, -273 degrees
What is the specific heat capacity of water
4.18 J g^-1 K^-1
What is the formula for temperature change
final temperature - initial temperature
what is the formula for energy change
mass x specific heat capacity x temperature change
what is specific heat capacity
the energy required to raise the temperature of 1 gram of a substance by 1K
What are 4 reasons why the recorded enthalpy change of combustion is different to the data value
- heat loss to surroundings
- incomplete combustion
- evaporation of substance from wick
- non standard conditions
Why are polystyrene cups often used when calculating enthalpy change of reaction
lightweight, waterproof, cheap snd insulating
